This invention relates to a laser spectroscopy method and apparatus for detecting and measuring the presence of individual constituent elements or molecules in a sample, particularly trace amounts of such constituents. More particularly, the invention relates to laser ablating a solid sample and creating a plasma from the ablated material. The ablated plasma is then subjected to another laser beam to excite-fluoresce an absorption state of a particular constituent. The relative amount of the particular constituent is determined by measuring the decay emission values of the excited absorption state and comparing it to decay emission values of samples of known trace constituent composition. The method is rapid and may be carried out in the atmosphere and under actual industrial situations. The apparatus is adaptable for field operation.
There are many situations where it is necessary or desirable to obtain substantially instantaneous trace constituent analysis of a sample material. For example, real time analysis of trace constituent contamination in the manufacture of copper cathodes enables continuous process control to ensure their elimination. Similar advantages can be obtained in the electrolytic manufacture of aluminum and its alloys. Such analysis also facilitates the purification and alloying of molten metals such as steel, cast iron, aluminum, brass, nickel alloys and such. Ideally such trace constituent analysis can be done without complex and time consuming sample preparation. Rapid trace constituent analysis is also desirable for detecting minute levels of heavy metals, dangerous substances or other contaminants in water, air or soil. Detecting impurities in thermal waste processes is also desirable for pollution control. To be practical, any such analysis should be done in the air or in the existing ambient atmosphere.
Recently, laser-induced breakdown spectroscopy (LIBS) has been used as a tool for real-time, in situ, primary composition and impurity analysis. In LIBS, a pulsed laser beam is focused onto a sample. This produces intense radiation that vaporizes, or ablates, a minute portion of the sample and forms a high temperature plasma from the ablated material. The excited atoms and ions in the plasma emit light that has a frequency characteristic of the emission states of the elements and molecules present in the sample. The composition of a sample is determined by analysis of the emission spectrum of the plasma as the atoms return to lower and ground states after laser radiation.
An advantage of the LIBS process is that a very small amount of material is ablated by the laser, typically only about 1 to 25 xcexcg for solid samples. Accordingly, for most applications, LIBS compositional analysis is considered nondestructive testing.
Another advantage of LIBS is that it is relatively easy to set up and is field deployable using modem portable pulsed lasers, fiber optic sensors, commercial photodetectors, emission spectrum analyzers, and such.
A serious disadvantage of LIBS is that it is inaccurate for determining the presence of trace amounts of elemental constituents, particularly those present in quantities less than about 100 parts per million (ppm). The generally weak spectral signals emitted by trace element constituents are difficult or impossible to separate from the background noise of a typical, complex, LIBS spectrum. Accordingly, the LIBS process has not been acceptable for doing real-time trace element analyses.
Experimental physicists have improved the detection limits of LIBS by probing the plasma created by LIBS with a second laser beam having a predetermined energy. The second beam is tuned to excite a fluorescent transition or absorption state of an ablated element or molecule of interest in the plasma. This process of laser induced fluorescence (LIF) has produced experimentally verifiable detection limits of about 10 ppm or greater for steel, for example. A drawback of this process has been that it must be practiced in a low pressure buffer gas (such as argon) or a vacuum to optimize emission yields. Creating and handling LIBS produced samples in a controlled, evacuated environment is time consuming, costly, and greatly inhibits the practical use of LIF in the field, foundry or factory.
Accordingly, there has been a long felt need for a method and apparatus for rapidly detecting trace amounts of elemental or molecular constituents in samples under practical conditions. In particular, there has been a need to detect the presence of trace constituents in the parts per billion range using a practical, field operative system.
In accordance with a preferred embodiment of the invention, a method and apparatus are provided for rapidly analyzing trace amounts of constituents in samples under ambient conditions. The invention has particular application to the identification of trace elements in copper cathode materials. By trace amounts herein, we generally mean amounts in the hundreds of parts per million or less.
The method comprises exposing a sample to a first laser beam which is of suitable wavelength and intensity to ablate a micro-specimen from the surface of a solid, or to completely vaporize a liquid and/or gaseous sample. The energy of the laser beam causes a plasma of the micro-specimen to form, which plasma has substantially the same composition as the sample.
Before this plasma decays, it is exposed to a second laser beam which is tuned to is have a wavelength and energy corresponding to an absorption state of a trace constituent of interest. This second laser radiation excite-fluoresces the selected absorption state and intensifies its decay emission.
The intensity value of the decay of the fluoresced element is measured and compared to a calibrated emission spectrum decay value.
In a preferred method in accordance with the invention, a sample is suitably located with respect to the analysis apparatus. A laser, such as a pulsed Nd:YAG laser, is focused on the sample and pulsed to ablate a micro-sample and form a micro-plasma. A tunable laser such as a dye laser or pumped optical parametric oscillator is used to pulse the plasma before it degrades substantially, thereby fluorescing an absorption state of a constituent of interest in the sample and increasing the emission spectrum.
In a preferred embodiment, means are provided to sequence the pulsing of the first and tuned lasers. A fiber optic is used to detect the emission radiation of the fluoresced plasma. The radiation output is fed to a monochromator or other spectrum analyzer the output of which is processed by a detector. The output of the detector is analyzed by a conventional computer processor and the background noise is subtracted. The net emission spectrum is compared to precalibrated spectrum concentration values for the constituent. Such calibrated concentration values are obtained by performing the subject method on samples having known amounts of the constituent being examined.
A preferred apparatus for this invention comprises a high energy pulsed laser for ablating a few micrograms of a sample and forming a plasma therefrom. A tunable laser is provided to excite-fluoresce a transition of a desired element or molecule of interest in the plasma. The high energy pulsed laser and tunable laser are sequenced by a delay generator.
The emission spectrum of an excite-fluoresced transition is sensed by a probe and fed to a monochromator. The output of the monochromator is analyzed by a sensor array. A portable computer is used to assemble the data and calculate the actual concentration of the constituent in the sample.